1. Field of the Invention
The present invention relates generally to implantable cardioversion-defibrillation (ICD) processes and systems, and more particularly to processes and systems that provide a programmable choice of a range of output capacitance values.
2. Description of the Background Art
Unlike existing external cardioversion-defibrillation systems which can deliver electrical countershocks of a wide variety of wave types and wave shapes from electronic circuitry that is located outside the human body, an implantable cardioversion-defibrillation (ICD) system must have electronic circuitry which is small enough to fit within an implantable device. This size limitations imposes significant constraints on the wave type and wave shape which can be delivered as an electrical countershock.
In all of the ICD systems available today, a truncated capacitive-discharge countershock is delivered by the implanted device to electrodes that are positioned in, on, or near the heart. To generate the truncated capacitive-discharge countershock, existing ICD systems use an internal battery system to charge a relatively small, but powerful, output capacitor system to a relatively high discharge voltage, the output of which is then discharged into the heart through the electrodes. After the output decays to a predetermined output voltage, or after a predetermined countershock duration has elapsed, the capacitive-discharge countershock is truncated and the remaining energy in the output capacitor system is dissipated within the ICD system. In existing ICD systems, the output wave shape of the countershock may be either a traditional monophasic waveform, or a biphasic waveform in which the polarity of the output discharge is reversed at some point during the discharge to present a second or reversed "phase".
In a given patient, there is a certain threshold energy required to achieve cardioversion, and usually a considerably higher value required to achieve defibrillation. Within the human population, these threshold values for countershocks delivered to implanted electrodes range widely, from approximately one joule to almost forty joules, depending upon such factors as the types and locations of the electrodes, the type of countershock, and the cardiac condition of the patient, for example. At the time an ICD system is implanted in a patient, the attending physician will empirically determine a minimum defibrillation threshold for the patient, and will program the charging voltages for the countershocks to be delivered as part of a therapy regimen within the range of maximum voltages allowed by the device.
Presently, there are three different ICD systems which have received device approval from the Federal Drug Administration, the PCD.RTM. device, available from Medtronic, Inc., of Minneapolis, Minn., the Cadence.RTM. device, available from Ventritex, Inc,. Mountain View, Calif., and the Ventak-P.RTM. device, available from Cardiac Pacemakers, Inc., St. Paul, Minn. The existing ICD systems are all capable of delivering a maximum countershock of up to 700 to 750 volts having a total energy of between 31 to 44 joules. In each of these ICD systems, the capacitor system consists of a pair of identical electrolytic capacitors that are connected in series and charged by a split winding transformer with one secondary winding connected to a first of the capacitors and the other secondary winding connected to a second of the capacitors. This arrangement overcomes the charging voltage limitations of the electrolytic capacitors that would otherwise limit the maximum charging voltage of the electrolytic capacitors to about 375 volts. Each of the pair of capacitors has the same capacitance value in the range of 280-360 microfarads. Because the pair of capacitors are discharged in series, there is a single total effective output capacitance of the capacitor system that is the equivalent of the series combination of the individual capacitance value of each capacitor, in this case one-half of the capacitance value of each of the individual capacitors. Thus, each of the existing ICD systems has a set effective output capacitance value that is someplace between 140-180 microfarads, depending upon the capacitance value of the identical pair of electrolytic capacitors used to make up the capacitor system.
In a capacitive-discharge system, the total energy stored in the capacitor can be determined by the equation: EQU E.sub.s =0.5 CV.sup.2 Eq.( 1)
where C is the output capacitance of the ICD system and V is the initial discharge voltage of the countershock. The portion of the stored energy (E.sub.s) that is actually delivered to the heart during a countershock can be determined by the equation: EQU E.sub.d =E.sub.s tilt Eq.(2)
where the tilt is a measure of the pulse duration of the truncated capacitive discharge usually expressed in terms of a percentage of the initial charging voltage (V.sub.i) to the final truncated voltage (V.sub.f) as defined by the equation: EQU tilt=(V.sub.i -V.sub.f)/V.sub.i Eq.( 3)
Alternatively, because the voltage decay of the capacitive discharge is a natural exponential decay, tilt may also be expressed directly as a function of the duration of the truncated capacitive discharge in terms of the equation: EQU tilt=V.sub.i (1-e.sup.-d/.tau.) Eq.(4)
where .tau. is the time constant of the ICD system as defined by the average resistance of the myocardium between the discharge electrodes (R) times the effective output capacitance of the ICD system (C).
It is generally accepted that achieving cardioversion or defibrillation with a countershock of the lowest possible energy brings benefits. One such benefit is that a lower energy delivered countershock minimizes patient hazard and discomfort during the countershock. Another benefit is that a lower energy delivered countershock may also lead to a decrease in the size of the battery and capacitor system required to deliver such a countershock, and, hence, to an ICD system having a smaller size, a longer life, or both. In addition, a capacitor system having a smaller effective capacitance reduces the energy drawn from the battery system in order to deliver the countershock, thereby resulting in a longer life for the battery system, a smaller size for the battery system, or a combination of both.
Because existing ICD systems have but one output capacitance value (C), the simplest and most customary way to reduce the energy (E.sub.d) of the countershock is to reduce the initial discharge voltage (V.sub.i), leaving the pulse duration of the countershock about the same. The problem which this technique engenders is that the effectiveness of both the cardioversion and defibrillation therapies are diminished by reducing the initial discharge voltage.
In the previously referenced co-pending applications entitled "Optimal Pulse Defibrillator", and "Short Pulse Cardioversion Method for Implantable Systems", it is disclosed that a higher percentage of successful conversions in both defibrillation and cardioversion can be achieved by preserving a high value for the initial discharge voltage, and, instead, diminishing the duration of the countershock pulse in order to reduce both the stored energy (E.sub.s) and the delivered energy (E.sub.d) of the countershock. A particularly favorable combination of initial voltage and pulse duration involves a value of the initial discharge voltage that falls between 500 and 800 volts, and a value of the countershock pulse duration that approximates the characteristic or chronaxie time of the heart and falls in the range from 1 to 4 milliseconds.
Thus, it is possible to achieve a higher success rate for cardioversion and defibrillation by truncating the capacitive discharge at an earlier point. The problem with this approach to reducing the energy of the countershock is that the ICD ends up wasting energy because there is more unused energy that remains in the capacitor system when it is truncated earlier. As a result, the energy delivered to the heart is now a smaller fraction of that initially stored in the capacitor system, and a larger fraction remains in the capacitor after discharge to be dissipated in the ICD system and thus wasted.
While the shorter pulse durations taught by the co-pending application may prove more successful with less output energy for the countershock, when shorter pulse durations are implemented in existing ICD systems the goal of reducing the overall energy requirements of the ICD system are not achieved due to the wasted energy that remains in the capacitor system after truncation. Consequently, it would be advantageous to provide an ICD system which can take advantage of the energy reductions afforded by shorter pulse duration cardioversion and defibrillation countershocks without wasting the energy that remains in the capacitor system after truncation of the capacitive-discharge countershock.